Gold Nanoparticle-Modified ITO Electrode for Electrogenerated Chemiluminescence: Well-Preserved Transparency and Highly-Enhanced Activity Zuofeng Chen and Yanbing Zu * Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China Supporting Information 1. The LOP ECL mechanism of the Ru(bpy) 2+ 3 /TPrA system. The ECL mechanism of the Ru(bpy) 2+ 3 /TPrA system has been studied intensively, and two emission routes have been proposed on the basis of the coreactant oxidation pathway. 1-5 When Ru(bpy) 2+ 3 concentration is relatively high, TPrA oxidation mainly proceeds via the catalytic route: Ru(bpy) 3 2+ e Ru(bpy) 3 3+ (E 0 ~2 V vs SCE) (1a) Ru(bpy) 3 3+ + TPrA Ru(bpy) 3 2+ + TPrA + (1b) But, in the presence of dilute Ru(bpy) 2+ 3 and concentrated TPrA, TPrA oxidation alters to mainly proceed following the direct oxidation route: TPrA e TPrA + (E 0 ~0.9 V vs SCE) (2) The product of the coreactant oxidation, TPrA cation radical (TPrA + ), will undergo a rapid decomposition, which leads to the formation of a highly reducing ECL intermediate, TPrA free radical (TPrA 3+ ); and then the excited state is produced by the energetic electron transfer between Ru(bpy) 3 and TPrA : * Corresponding author. Tel.: +852 28598023; fax: +852 28571586. E-mail address: ybzu@hku.hk (Y. Zu).
TPrA + TPrA + H + (3) Ru(bpy) 3+ 3 + TPrA Ru(bpy) 2+* 3 + P1 (4) where TPrA + = Pr 3 N +, TPrA = Pr 2 NC HCH 2 CH 3, and P1 = Pr 2 NC + HCH 2 CH 3. Recently, a novel ECL route, which involves the intermediacy of the TPrA cation radical, was proposed: 6 Ru(bpy) 2+ 3 + TPrA Ru(bpy) + 3 + P1 (5) Ru(bpy) + 3 + TPrA + Ru(bpy) 2+* 3 + TPrA (6) Since no oxidation of Ru(bpy) 2+ 3 is required in this ECL route, the emission can be generated at a potential below V vs SCE. Evident low-oxidation-potential (LOP) ECL signals have been observed at freshly polished glassy carbon (GC) and fluorosurfactant-modified gold electrodes. 2,6-10 It is noted that, although the LOP ECL could be produced at a GC electrode, the overlapping of the LOP emission signal with the conventional ECL peak at a higher oxidation potential made it difficult to measure the LOP ECL intensity accurately. Instead, a fluorosurfactant-modified gold electrode is more suitable for producing the LOP ECL signal. References (1) Leland, J. K.; Powell, M. J. J. Electrochem. Soc. 1990, 137, 3127. (2) Zu, Y.; Bard, A. J. Anal. Chem. 2000, 72, 3223. (3) Kanoufi, F.; Zu, Y.; Bard, A. J. J. Phys. Chem. B 2001, 105, 210. (4) Zu, Y.; Bard, A. J. Anal. Chem. 2001, 73, 3960. (5) Honda, K.; Yoshimura, M.; Rao, T. N.; Fujishima, A. J. Phys. Chem. B 2003, 107, 1653. (6) Miao, W.; Choi, J. P.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 14478. (7) Zheng, H.; Zu, Y. J. Phys. Chem. B 2005, 109, 12049. (8) Zheng, H.; Zu, Y. J. Phys. Chem. B 2005, 109, 16047. (9) Li, F.; Zu, Y. Anal. Chem. 2004, 76, 1768.
(10) Zu, Y.; Li, F. Anal. Chim. Acta 2005, 550, 47. 2. Experimental. Chemicals Tris(2,2'-bipyridyl)ruthenium(II) dichloride hexahydrate (Ru(bpy) 3 Cl 2 6H 2 O, min 98%), tri-n-propylamine (TPrA, 98%), and Zonyl FSO-100 (F(CF 2 CF 2 ) 1-7 CH 2 CH 2 O(CH 2 CH 2 O) 0-15 H), hydrogen tetrachloroaurate(ш) trihydrate (HAuCl 4 3H 2 O), trisodium citrate, NaBH 4, (3- aminopropyl)triethoxysilane (APTS) were purchased from Sigma-Aldrich. ITO electrodes (ITO-coated glass, R s = 4 8 ohms) were obtained from Delta Technologies, Limited. Other chemicals were analytical reagent graded and used as received. All solutions were prepared with deionized water (Milli Q, Millipore). The ph of the phosphate buffer solution (0.15 M) containing TPrA was adjusted with concentrated NaOH or phosphoric acid. Apparatus Transmission electron microscopy (TEM) images were taken using a Philips microscope (Tecnai 20) operated at an acceleration voltage of 200 kv. Scanning electron microscope (SEM) images were obtained at Leo 1530 FEG. UV-vis data were recorded on a Hewlett-Packard 8453 diode-array UV-vis spectrophotometer. The three-electrode system consisted of a bare or modified ITO electrode with an exposed geometric area of ca. 4 cm 2 as a working electrode, a coiled Pt wire as counter electrode, and a saturated calomel reference electrode (SCE) separated from the working cell by a salt bridge. All of the electrochemical measurements were performed on a CHI 760B electrochemical workstation (Chenhua Instruments, Shanghai). The ECL signal was measured with a photomultiplier tube (PMT, Hamamatsu R928) installed under the electrochemical cell. A voltage of 600 V was supplied to the PMT with the Sciencetech PMH-02 instrument (Sciencetech Inc., Hamilton, Ontario, Canada). Gold nanoparticles synthesis Colloidal GNPs with average diameters of 4 nm, 12 nm and 40 nm were prepared following the literature procedures (references as indicated in the text). All glassware used for preparation of Au colloids were thoroughly washed with freshly prepared aqua regia (HNO 3 : HCl = 1 : 3), rinsed extensively with deionized and ultrahigh purity water sequentially, and then dried in an oven
at 100 C for 2-3 hour. A 60 ml solution of 1 % HAuCl 4 was brought to a vigorous boil with stirring in a round-bottom flask fitted with a reflux condenser, and then different amounts of 5% citrate was added to the stirring and refluxing HAuCl 4 solution (for 12 nm and 40 nm GNPs, 1 ml and 0.12 ml of citrate were used, respectively; for 4 nm GNPs, 0.12 ml of citrate was added, followed one min later by addition of 0.12 ml of 0.375% NaBH 4 in 5% citrate). The solution was maintained at the boiling point with continuous stirring for about 15 min. After the solution was allowed to cool to room temperature, the suspension was stored at 4 C until further use. The TEM specimens were prepared by depositing an appropriate amount of GNPs onto the carbon-coated copper grids, and excess solution was wicked away by a filter paper. The grid was subsequently dried in air. Other procedures A polycrystalline gold electrode of 2-mm-diameter was used in the control experiments. Prior to the experiments, the bare gold electrode was wet polished with 5 µm Al 2 O 3 powders to obtain a mirror surface followed by sonication in distilled water for 10 seconds, and then was subjected to repeated scanning in a wide potential range in 0.1 M H 2 SO 4 solution until reproducible voltammograms were obtained. The FSO-modified gold electrode (FSO-Au) was prepared by dipping the pretreated gold electrode into a 0.5 wt.% FSO aqueous solution for 10 min, followed by thoroughly rinsing with distilled water. UV-vis spectra of GNP colloidal solutions and GNP-modified ITO electrodes were measured with distilled water and air as reference, respectively. Cyclic voltammetry (CV) was performed with a scan rate of 100 mv/s. The electrolyte solutions were deaerated by bubbling high purity (99.995%) N 2, and a constant flow of N 2 was maintained over the solution during the ECL measurements. All potentials reported in this paper are referred to the SCE. All experiments were performed at room temperature 20 ±1 C.
3. UV-vis spectra of the GNP colloidal solutions and TEM images. (a) 1.2 (b) 1.4 1.2 (c) Figure S1. UV-vis spectra of the colloidal solutions of 4 nm (a), 12 nm (b) and 40 nm (c) GNPs. Inset, TEM images of the colloidal GNPs. 4. TPrA oxidation and corresponding LOP ECL at the FSO-modified polycrystalline gold electrode. 15 I ecl (a.u.) 10 5 0 500 I (µa) 0-500 -1000 1.41.2- E (V, vs. SCE) Figure S2. Cyclic voltammograms and corresponding ECL curves for 1 µm Ru(bpy) 2+ 3 and 10 mm TPrA in 0.15 M phosphate buffer solution (ph 7.5) at a polycrystalline gold electrode(solid line) and an FSO-Au electrode (dashed line). Potential scan rate, 100 mv/s. For comparison purpose, the CV and ECL profiles have been corrected according to the geometric area of the ITO electrode.